1. Field of the Invention
The invention relates generally to radio frequency receivers, and more specifically to multiple band global positioning system (GPS) receivers used for navigation.
2. Description of the Related Art
GPS satellites transmit data at two radio frequency (RF) system carrier frequencies: 1575.42 MHz (L1) and 1227.6 MHz (L2). GPS data from both carriers can be used to increase the position accuracy, and to provide carrier selectivity in case of interference or jamming of one of the carriers.
A GPS receiver designed to receive the L1 and/or L2 carriers requires a method for receiving both signals simultaneously or efficiently switching between the signals. One solution is to duplicate all receiver parts and functions for the L1 and L2 bands. However, for low-power portable receivers, it is desirable to integrate the L1 and L2 functions as much as possible, to minimize the number of RF functions and power dissipation.
It has been known for L1/L2 receivers to use parallel RF paths and/or RF switching of the input and/or local oscillator (LO) signals. For example, U.S. Pat. No. 5,883,597 discloses an L1/L2 GPS receiver in which the LO is switched between three frequencies to select xe2x80x9cL1 only,xe2x80x9d xe2x80x9cL2 onlyxe2x80x9d or xe2x80x9cL1 and L2.xe2x80x9d However, this requires the LO to be tunable over a very wide frequency range of about 696 MHz, from approximately 1054 MHz to 1750 MHz, which makes on-chip integration difficult. Further, due to practical design limitations, this may require switching between two or three tuned oscillators, which may result in excessive power consumption for multiple voltage controlled oscillators (VCOs). Also, in the xe2x80x9cL1 and L2xe2x80x9d mode, this receiver may suffer a 3 dB noise penalty due to image noise. Switching of the LO signal may also require resynchronization of tracking loops, which reduces receiver response time for time sensitive applications.
U.S. Pat. No. 5,678,169, for example, discloses an L1/L2 receiver in which the VCO and LO frequency is fixed exactly halfway between the L1 and L2 carriers, as in the xe2x80x9cL1 and L2xe2x80x9d mode of the above referred-to receiver. This receiver uses switched L1 and L2 filters which eliminate the problem of the 3 dB image noise. However, this receiver may not be capable of true simultaneous L1 and L2 detection, since the L1/L2 selection is done by RF switches before the mixer.
U.S. Pat. Nos. 5,040,240 and 5,736,961, for example, disclose L1/L2 receivers which use parallel RF paths for the downconversion. U.S. Pat. No. 5,040,240 uses a common VCO with a series of different dividers and multipliers for the L1 and L2 downconversions. However, due to the duplication of RF functions, these methods are not optimum for high integration and low-power.
Therefore, those concerned with the development and use of improved dual frequency carrier signal receiver systems and methods have recognized the need for improved systems and methods for enabling simultaneous dual frequency capabilities without requiring radio frequency switches or local oscillator switching.
Accordingly, the present invention fulfills these needs by providing efficient and effective systems and methods for simultaneously receiving or switching between dual frequency carrier signals in a highly integrated, low power receiver.
Briefly, and in general terms, the present invention provides a system and method for simultaneously receiving or switching between dual frequency carrier signals.
By way of example, and not by way of limitation, the present invention provides a new and improved system for simultaneously receiving or switching dual frequency carrier signals, without local oscillator switching or radio frequency switches.
More particularly, the present invention includes a sub-harmonic frequency generator, which may include a sub-harmonic VCO, with different harmonics of the sub-harmonic frequency VCO providing the local oscillator signals for the L1 and L2 carriers. Downconversion in the sub-harmonic frequency generator or a first mixer then produces two intermediate frequencies (IF) for the L1 and L2 carriers. The VCO frequency and harmonic orders may be chosen such that the difference between these two IF signals is twice the desired final IF. The final IF may be obtained through a second mix in a second mixer with an LO signal that is halfway between the L1 and L2 IF frequencies. Since these IF signals generated in the first mixer are on either side of the LO frequency they can be separated by having the second mixer be an image reject mixer. The image reject mixer can be used to receive L1 and L2 simultaneously using both its outputs, or to switch between L1 and L2. The selection is accomplished by interchanging the xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d LO input signals of the second IR mixer. Since this switching is done at a lower IF frequency it does not cause unlocking of the phase locked loop (PLL) or the receiver tracking loop.
This receiver architecture is chosen to minimize power dissipation, while optimizing integration and performance. Operation of an on-chip integrated VCO at a frequency three to four times lower than the L1/L2 RF carriers saves power in the VCO and PLL. Switching at the IF frequency consumes less power compared to RF or LO switching, and does not degrade the receiver noise figure. RF switches introduce front-end loss which degrades the receiver noise figure. Only one external split band filter is required at the front end to reject the first image frequencies for the L1 and L2 downconversion. The second image is rejected by the image reject function of the second mixer. There is no 3 dB degradation for simultaneous L1/L2 herein.
A single fixed frequency VCO eliminates the need of LO switching, and eliminates the need of RF switches, while still providing simultaneous L1 and L2 capability.
Although the preferred embodiment described is an L1/L2 GPS receiver, the systems and methods described herein can be used for any dual frequency RF receiver.